The present invention relates to an individual ignition type ignition coil device for internal combustion engine, which is provided for each spark plug and directly connected to each spark plug for use.
In recent years, an individual ignition type ignition coil device for engine which is inserted in a plug hole of the engine and individually and directly connected to each plug is being developed. This kind of ignition coil device does not need a distributor, as a result, the ignition device does not cause drop of energy supplied to an spark plug due to the distributor, high voltage cords, etc. Further, since the ignition coil can be designed without considering the drop of ignition energy, the ignition coil is evaluated as a device that it is possible to make the ignition coil small in size by reducing the coil volume and improve a part installation space of an engine room by exclusion of the distributor.
Of such individual ignition type ignition coil devices, an ignition coil device of type in which at least a part of a coil portion is inserted in a plug hole and installed is called a plug hole interior equipped type.
Further, since the coil portion is inserted in the plug hole, it is made long and slender like a pencil shape and called a pencil coil, and a center core (which is a magnetic path of an iron core, and a lamination of a lot of silicon steel sheets), a primary coil and secondary coil are inserted inside a long and slender cylinder type coil case. The primary coil and secondary coil are wound around bobbins, respectively, and the bobbins are arranged concentrically with the center core. Inside the coil case containing such primary and secondary coils, insulating resin is injected and hardened or insulating oil is sealed, thereby to secure insulation. As relevant prior arts, for example, JP A 8-255719, JP A 9-7860, JP A 9-17662, JP A 8-93616, JP A 8-97057, JP A 8-144916, JP A 8-203757 and JP A 9-167709 are raised.
Of this kind of individual ignition type ignition coil devices, an ignition coil device of type in which insulating resin such as epoxy resin is injected into the coil case and hardened does not need to take countermeasures to sealing of oil as in the insulating oil sealing, further, components such as the center core, bobbins, coils, etc. can be fixed only by immersing them into the insulating resin, so that it is evaluated as a device that the fixing of those parts is simple compared with the insulating oil sealing type and it is possible to simplify the whole device and facilitate its handling.
However, since the insulating resin injected (filled in) between components of the ignition coil device has thermal stress (heat shock) applied thereon on the basis of difference in linear expansion coefficient between the components, it is necessary to take countermeasures to cracking due to heat shock and boundary separation between the components. Particularly, the individual ignition type ignition coil device of type in which it is inserted in a plug hole of the engine is exposed to a severe temperature condition (-40.degree. C. to -130.degree. C. ) and the insulating resin is necessary to resist the heat shock.
Occurrence of cracks causes the following dielectric breakdown. For example, in the case of a type in which a center core, secondary coil and primary coil are equipped inside in turn (in the case of a so-called inside secondary coil structure), when an air gap occurs by the crack between the secondary coil and the center core and between the secondary coil and the primary coil, where potential difference exists, so-called electric filed concentration that field intensity becomes extremely large occurs and dielectric breakdown occurs.